The invention is generally related to floating offshore structures and more particularly to the centerwell arrangement of a spar type hull.
There are a number of spar hull designs available in the offshore drilling and production industry. These include the truss spar (FIG. 1C), classic spar (FIG. 1B), and cell spar (FIG. 1A). The term spar hull structure described herein refers to any floating structure platform, which those of ordinary skill in the offshore industry will understand as any floating production and/or drilling platform or vessel having an open centerwell configuration.
A spar hull is designed to support a topsides platform and riser system used to extract hydrocarbons from reservoirs beneath the seafloor. The topsides supports equipment to process the hydrocarbons for export to transport pipelines or to a transport tanker. The topsides can also support drilling equipment to drill and complete the wells penetrating the reservoir. The product from these wells is brought up to the production platform on the topsides by risers. The riser systems may be either flexible or steel catenary risers (SCR's) or top tensioned risers (TTR's) or a combination thereof.
The catenary risers may be attached at any point on the spar hull and routed to the production equipment on the topsides. The routing may be on the exterior of the hull or through the interior of the hull. The TTR's are generally routed from wellheads on the seafloor to the production equipment on the topsides platform through the open centerwell.
These TTR's may be used for either production risers to bring product up from the reservoir or as drilling risers to drill the wells and provide access to the reservoirs. In some designs where TTR's are used, either buoyancy cans or pneumatic-hydraulic tensioners can support (hold up) these risers. When buoyancy cans are used, the buoyancy to hold up the risers is supplied independently of the hull and when tensioners are used these tensioners are mounted on the spar hull and thus the buoyancy to hold up the risers is supplied by the spar hull. In either method of supporting the risers, TTR's are generally arranged in a matrix configuration inside an open centerwell. The spacing among the risers in this centerwell location is set to create a spacing among the risers that allows manual access to the production trees mounted on top of the risers.
The spar type structure which supports the topsides comprises a hard tank and other structural sections such as a truss and a soft tank or the hull can be completely enclosed as a cylinder. The hard tank supplies the majority of the buoyancy to support the hull structure, risers, and topsides platform. The hard tank is compartmentalized into a plurality of chambers among which the ballast can be shifted to control the hull's stability.
The open configuration in the center of the hard tank forms an open volume in the center of the hard tank referred to as the centerwell.
Since 1997, fifteen spar type structures have been constructed and installed. The spar type structures currently in operation are the cell spar, classic spar, and truss spar (shown in FIG. 1). The cell spar is constructed from a number of closed cylindrical cells to form the hull and supply most of the buoyancy. The classic and truss spar have a common component, typically referred to as the “hard tank”, which supplies most of the buoyancy.
The primary advantage of the spar type structure is its low motions that minimize damage to riser and mooring systems and allow top tensioned risers with dry trees to be used for production. Topsides are supported on top of the spar using structural legs that adjust the height of the lower deck to avoid contact with waves. An open space in the center of the topsides, which is coaxial with the centerwell, is referred to as the wellbay (FIGS. 2, 3).
The riser arrangements are generally based on versions of catenary risers and top tensioned risers (TTR's). TTR's can be further categorized as production TTR and drilling riser TTR. The risers enter the spar topsides through the wellbay extending up from the centerwell. On all existing spar hulls the centerwell opening is square (FIGS. 2A, 3A, 4A, 6A, 7A). The size of the opening varies, depending on the number of risers coming up through the wellbay.
On conventional spars that have a drilling facility, the drilling rig and equipment are positioned on the top of the drilling deck above the open wellbay (FIG. 5A). The riser slots in the wellbay are arranged in a matrix configuration (FIG. 4A). The wells are drilled through these slots using the drilling riser. After the drilling phase of the well, the drilling rig is used to install or “run” the production TTR in the riser slot and connect it to the production manifold through the dry production tree and flexible jumper.
Subsequent to the initial installation of risers, there are interventions or “work-overs” into the oil and natural gas reservoir through the production TTR's for various reasons such as well stimulation and control. The drilling rig is also used for this operation. Consequently, drilling is suspended and the drilling rig is dedicated to work-over activities. The re-allocation of the drilling facilities causes a delay in drilling activity.
In order to access each well slot, the rig has to be positioned or “skidded” above the appropriate slot and therefore must be skidded in two horizontal directions as illustrated in FIG. 6A. Because of the weight of the drilling rig and the spans required to traverse the wellbay, the rig support structure is constructed of heavy structural beams, referred to as “skid beams”.
In the traditional wellbay of currently operating spar type structures the risers/TTR's are positioned in a matrix format in the internal part of the wellbay (FIGS. 3A, 7A). At the top of these riser/TTR's is a dry tree which is connected to a manifold on the process equipment through a flexible jumper line. Fluids and gases from the reservoir flow up these risers/TTR's through the dry tree and flexible jumper connection and into the manifold. In the matrix layout, these jumpers sometimes cross over each other (FIG. 7A), which makes their design very difficult and in some cases limit the functionality of the design.
As mentioned above, all currently operating spar type structures equipped with drill rigs involve the installation of drilling equipment including its support structure and drill rig derrick mounted on top of the upper deck of the spar. This subjects the spar to large wind load areas at a high elevation and induces large overturning moments on the spar hull. In order to counteract these moments, heavy ballast is installed in the keel of the spar. This ballast must be supported by the buoyancy chambers in the spar, which increases the size of the spar hull.
Currently operating spar type structures using the matrix configuration completely utilize the wellbay and prevent this space from being used for other purposes.